Many of us CNC operators are in the habit of always climb milling but sometimes there can be a preference towards conventional milling, especially on smaller machines.
To gain better knowledge about when to use either, a quick definition of the differences.
Climb milling refers to the situation where the direction of cut and rotation of the cutter combine and try to “climb” the mill up over (hence it’s called “climb” milling) or away from the work. This result is a prime surface finish.
Climb Milling is overall the best way to machine on a rigid machine since it reduces the load from the cutting edge, leaves a better surface finish, and improves tool life. During Conventional Milling, the cutter tends to dig into the work piece and may cause the part to be cut out of tolerance.
However, though Climb Milling is the preferred way to machine parts, there are times when Conventional Milling is the necessary milling style especially on a lighter machine. Another example is if your machine does not counteract backlash. In this case, Conventional Milling should be implemented. In addition, this style should also be utilised on casting, forgings or when the part is case hardened (since the cut begins under the surface of the material).
You can experiment on a mill by cutting both ways and you will notice that climb milling is a lot easier and the outcome is a better surface finish. However as we will explore below, there are exceptions to this rule. especially when using a carbide 3 flute endmill in aluminium
Advantages of Conventional Milling (Up Milling):
- The width of the chip begins zero and increases as the cutter finishes slicing.
- The tooth meets the workpiece at the base of the cut.
- Forces directed upwards are created and tend to lift the workpiece during face milling.
- More energy is needed to conventional mill than climb mill.
- Surface isn't the best because teeth carry chips upwards and drop them in front of the cutter. The chips are recut a lot. However, flood cooling helps.
- Tools erode faster than normal with climb milling.
- Conventional milling best used for rough surfaces.
- Deflection of tools during Conventional milling is often parallel to the cut (see the section on Tool Deflection for more).
Advantages of climb milling:
- The width of the chip begins at the widest point and decreases.
- The tooth contacts the workpiece at the top of the cut.
- Chips are dropped behind the cutter–less recutting.
- Eroding is reduced, tools can last up to 50% longer.
- The finish on the surface is better because there is less recutting.
- It requires less power.
- There is a downwards exertion of force during face milling. This makes the workholding and fixtures easier. The downward force could also help lessen chatter in thin floors. This is because it helps brace them against the surface below.
- Climb milling makes work easier.
- However, it can bring about chipping when milling materials rolled with heat due to the tough layer on the surface.
- Deflection of tools during climb milling are often perpendicular to the cut. This may cause a decrease or increase in the width of cut and ultimately affect accuracy.
Climb Milling issues:
The problem with climb milling is that it can run into problems with backlash if cutter forces are strong enough. The issue is that the table will likely be pulled into the cutter when climb milling. In the case of a backlash, there is room for pulling relative to the quantity of backlash. If there is enough backlash with the cutter functioning at capacity, there could be breakage and potential injury due to flying shrapnel.
On this grounds, a lot of shops simply disallow climb milling totally on any hand-operated machines that have backlash. Some machines are even fitted with a “backlash eliminator” whose primary purpose is to enable climb milling.
A way to think of it is to examine the idea of chip load. This shows how much material each tooth of the endmill is trying to cut. Typical values for finish might read 0.001 to 0.002″ per tooth. For roughing work, it might increase to 0.005″. Climb milling may grab the table and slam the work into the cutter at full amount of backlash in the worst cases, at the time when a single tooth is cutting. At this point, the backlash can be added to the chip load to note what the new effective chip might be. For instance, you're roughing 0.005″ per tooth and have 0.003″ backlash. Your chip load will soar to 0.008″ in the worst case. It's not so terrible, but it is a strain. So, suppose you have a less advanced machine with 0.020″ of backlash and running a 0.005″ chip load. At the worst, your chip load will soar to 0.025″, which might break the endmill and is very dangerous.
Another point to examine is if cutting forces are strong enough to drag the table through the backlash in the first place. A lot will depend on the exact cutting scenario together with your machine. If low friction linear way machine is available, it can grab easily. If there are a lot of iron in the table, and you are probably running with the gibs tightened a bit, it’ll be harder. There are ways to calculate the cutter force, but generally, smaller end mills, less depth of cut, lower feeds, and lower spindle speed will all tune down the cutting force and make it less likely for the cutter to drag the backlash out of the table and make things difficult.
Generally, CNC machines should not have any obvious backlash so these causes more concerns on manual machines and lighter gantry style CNC routers
Should i always climb mill?
So far, you're probably going to be inclined to always climb mill. Besides, the outcome is a better surface finish, requires less energy, and the possibility to deflect cutter is less. On the other hand, manual machinists are often instructed never to climb mill because of the hazards involved in doing so on a machine that has backlash.
Some pointers for when to consider not climb milling
- Small and light machines, CNC routers, Gantry mills and entry level China CNC machines can sometimes be more stable in the cut when using conventional milling.
– When you want to cut half the cutter diameter or less, you need to climb mill. (assuming your machine has low or no backlash and it is safe to do so!).
– Up to 3/4 of the cutter diameter, it doesn’t matter which way you cut.
– Cutting from 3/4 to 1x the cutter diameter, you should favor conventional milling.
This is due to the fact that cutter geometry forces equal amount of negative rake cutting for those heavy 3/4 to 1x diameter cuts.
Consider Conventional Milling for Finish Passes
This one is counterintuitive for a lot of machinists who have been instructed that climb gives a better finish than conventional. All other things being equal, that is true, but all other things are not always equal!
The problem is that deflection tampers with surface finish too. If the vector is almost parallel to the path, you can think that the portion of the vector that pushes it “off parallel” is very little. Therefore, the tool will have a small tendency to deflect and put waves on the wall you’re finishing. Note that this may be particularly essential in thin wall work where the walls aren't strong!
You should opt to work with conventional milling for the finish pass if you’re at all deflection challenged
At the least, one should try reduce too much depth of cut when climb milling lest it cause deflection.
When deflection is to be minimized, use no more than 30% of the diameter of the cutter for conventional milling and 5% for climb milling
Managing deflection properly can help you circumvent the need for an extra finishing cut, which saves money and time.